I argue that if a subject's acquaintance with an object is necessary for him to think about and refer to the object, then the content of his thought cannot be a set of metaphysically possible worlds. Acquaintance resets what possibilities there are; it affects the powers of representation, and does not only limit the range of possibilities. If acquaintance restricts what a subject can think about, the theorist cannot specify what possibilities are open to the subject simply in terms of possible worlds.